Cooking oil is plant, animal, or synthetic fat used in frying, baking, and other types of cooking. It is also used in food preparation and flavouring not involving heat, such as salad dressings and bread dips, and in this sense might be more accurately termed edible oil. Cooking oil is typically a liquid at room temperature, although some oils that contain saturated fat, such as coconut oil, palm oil and palm kernel oil are solid. There are a wide variety of cooking oils from plant sources such as olive oil, palm oil, soybean oil, canola oil (rapeseed oil), corn oil, peanut oil and other vegetable oils, as well as animal-based oils like butter and lard. Oil can be flavored with aromatic foodstuffs such as herbs, chillies or garlic.
Heating an oil changes its characteristics. Oils that are healthy at room temperature can become unhealthy when heated above certain temperatures. When choosing cooking oil, it is important to match the oil’s heat tolerance with the cooking method. Palm oil contains more saturated fats than canola oil, corn oil, linseed oil, soybean oil, safflower oil, and sunflower oil. Therefore, palm oil can withstand the high heat of deep frying and is resistant to oxidation compared to highly unsaturated vegetable oils. Palm Oil has been increasingly incorporated into food by the global commercial food industry because it remains stable in deep frying or in baking at very high temperatures and for its high levels of natural antioxidants. All oils degrade in response to heat, light, and oxygen. To delay the development of rancidity, a blanket of an inert gas, usually nitrogen, is applied to the vapor space in the storage container immediately after production, a process called tank blanketing. In a cool, dry place, oils have greater stability, but may thicken, although they will soon return to liquid if they stand at room temperature. To minimize the degrading effects of heat and light, oils should be removed from cold storage just long enough for use. Refined oils high in monounsaturated fats, such as macadamia oil, keep up to a year, while those high in polyunsaturated fats keep about six months. Rancidity tests show that the life of walnut oil is about 3 months, a period considerably shorter than the “best before” duration shown on labels. In contrast, saturated oils, such as avocado oil, have relatively long shelf lives and can be safely stored at room temperature, as their lower polyunsaturated content facilitates stability.
Cooking oil can be recycled. It can be used as animal feed, directly as fuel, and to produce biodiesel, soap, and other industrial products. In the recycling industry, used cooking oil recovered from restaurants and food-processing industries (typically from deep fryers or griddles) is called recycled vegetable oil (RVO), used vegetable oil (UVO), waste vegetable oil (WVO), or yellow grease. Yellow grease is used to feed livestock, and to make soap, make-up, clothes, rubber, detergents, and biodiesel fuel. Used cooking oil, besides being converted to biodiesel, can be used directly in modified diesel engines and for heating. Grease traps or interceptors collect fats and oils from kitchen sinks and floor drains which would othewise clog sewer lines and interfere with septic systems and sewage treatment. The collected product is called brown grease in the recycling industry. Brown grease is contaminated with rotted food solids and considered unsuitable for re-use in most applications.